Prostate cancer is the most commonly diagnosed cancer and the second leading cause of cancer death in males. There is growing evidence that cancers, including prostate cancers, are diseases of stem cells. To understand how to control cancers we must understand the properties of stem cells. Stem cells, including cancer stem cells, reside in a protective environment, known as the niche, that maintains them in an undifferentiated pluripotent state. Little is known about any mammalian niche. In mouse prostate, the stem cell niche is located in the proximal region of the duct and correlates with high vascularity of this region. The role of blood vessels and blood vessel endothelial cells in maintaining the niche has not been investigated. We hypothesize that prostatic stem cells play a critical role in determining the vascularity of the proximal prostate and the high vascular density in the proximal region may be a critical factor in establishing the stem cell niche in that region. We will examine the importance of angiogenic signals from prostatic stem cells in determining the vascularity of the proximal region and its ability to harbor stem cells. We will also examine signals from the endothelial cells that support stem cell function. Aim 1 will examine the epithelial stem cell signals that regulate prostatic vascular growth. In this aim, we will examine if alterations in angiogenic signals from stem cells affect proximal vascularity and prostate stem cell content. We will use a model system in which prostate cells and urogenital sinus mesenchyme are co-inoculated under the renal capsule of male athymic mice, where they form a prostate-like organ with ducts arranged in a distinct proximal-distal axis. We transfect proximal prostate cells with small interfering RNAs for VEGF-A, VEGF-C, or angiopoietin-2 using a lentiviral vector to diminish production of the corresponding angiogenic molecules. The transfected cells will be inoculated under the renal capsule, and the effect of diminished angiogenic factor production on the growth of the prostate, vascularity of the proximal region, and content of cells expressing stem cell markers will be evaluated. These experiments will determine which angiogenic signals from the proximal cells are important for proximal prostate vascularity and if angiogenic signals from Sca-1Hi stem cells are more potent than signals from the remaining cells. Aim 2 will explore the importance of endothelial cell signals in supporting stem cell function. We will determine if endothelial cells are able to support prostate regeneration by Sca-1Hi stem cells using the renal capsule assay. We will also use microarray analysis to determine signaling pathways used by endothelial cells in the stem cell niche. The experiments will provide novel information on the role of endothelial cells in the prostate stem cell niche that will also be relevant to the prostate tumor stem cell niche.